How can I download HTML source in C#

You can download files with the WebClient class:

using System.Net;

using (WebClient client = new WebClient ()) // WebClient class inherits IDisposable
{
    client.DownloadFile("http://yoursite.com/page.html", @"C:\localfile.html");

    // Or you can get the file content without saving it
    string htmlCode = client.DownloadString("http://yoursite.com/page.html");
}

basically:

using System.Net;
using System.Net.Http;  // in LINQPad, also add a reference to System.Net.Http.dll

WebRequest req = HttpWebRequest.Create("http://google.com");
req.Method = "GET";

string source;
using (StreamReader reader = new StreamReader(req.GetResponse().GetResponseStream()))
{
    source = reader.ReadToEnd();
}

Console.WriteLine(source);

The newest, most recent, up to date answer
This post is really old (it's 7 years old when I answered it), so no one of the other answers used the new and recommended way, which is HttpClient class.

---

HttpClient is considered the new API and it should replace the old ones (WebClient and WebRequest)

string url = "page url";
HttpClient client = new HttpClient();
using (HttpResponseMessage response = client.GetAsync(url).Result)
{
   using (HttpContent content = response.Content)
   {
      string result = content.ReadAsStringAsync().Result;
   }
}

for more information about how to use the HttpClient class (especially in async cases), you can refer this question

---

NOTE 1: If you want to use async/await

string url = "page url";
HttpClient client = new HttpClient();   // actually only one object should be created by Application
using (HttpResponseMessage response = await client.GetAsync(url))
{
   using (HttpContent content = response.Content)
   {
      string result = await content.ReadAsStringAsync();
   }
}

---

NOTE 2: If use C# 8 features

string url = "page url";
HttpClient client = new HttpClient();
using HttpResponseMessage response = await client.GetAsync(url);
using HttpContent content = response.Content;
string result = await content.ReadAsStringAsync();